Repairing the nervous system after injury, neurodegenerative disease, or dysfunctional development represents a significant challenge to biomedical research. Understanding how newly born neurons are guided by signals in their environment to form the correct neuronal morphology and synaptic connections is key to developing therapies to repair these disease states. Our current understanding of how neurons develop their specific shape and set of synapses is primarily the result of cell culture studies, where developing neurons are not exposed to the normal set of extracellular cues. This proposal combines mouse genetics, time-lapse imaging, and phenotypic analysis to understand how neuronal polarization and synaptogenesis are orchestrated in the context of the developing tissue. The atypical cadherin Fat3 offers a unique molecular entry point to investigate the relationship between synapse formation and neuronal morphology. Fat3 is expressed in the amacrine and retinal ganglion cells of the retina. Genetic deletion of fat3 in mice results in ectopic synapses and disrupted morphology in amacrine cells. The events that underlie this phenotype are unknown. This proposal will test the central hypothesis that Fat3 mediates cell-cell interactions that position and shape axons and dendrites, which in turn restricts synaptic location. To this end I will investigate the role of Fat3 in linking extracellular cues to neuronal polarity and synaptogenesis, and then ask if Fat3 plays a similar role in the development of other neurons. Specific Aim 1 is focused on investigating cellular events that lead to the correct development of amacrine cells, and then determining what events are Fat3-dependent. Establishment of a novel time-lapse retinal slice imaging method will facilitate the analysis of single amacrine cells developing in their natural context in mammals. Additionally, this aim seeks to understand the relationship between neuronal polarization and synaptogenesis. Specific Aim 2 is directed at understanding the role of Fat3-mediated cell-cell interactions with other neurons that controls the development of retinal ganglion cell axonal and dendritic morphology. Deletion of fat3 from specific populations of retinal ganglion and amacrine cells, followed by phenotypic analysis of the morphology of the resulting retinal ganglion cell axon and dendrites will provide a clear understanding of how cell-cell contacts mediate development. Collectively, the work described here will provide novel insight into amacrine and retinal ganglion cell development. Specifically, the results will answer key questions about the relationship between neuronal morphology and synapse formation in the context of the extracellular environment, as well as the role of cellular interactions in forming neuronal circuit.